Rate Coefficients for OH + NO (+N2) in the Fall-off Regime and the Impact of Water Vapor

The termolecular, association reaction between OH and NO is a source of nitrous acid (HONO), an important atmospheric trace gas. Rate coefficients for the title reaction as recommended by evaluation panels differ substantially at the temperatures and pressures that prevail in the Earth’s boundary layer where the reaction is in the fall-off regime between low- and high-pressure limiting rate coefficients. Using pulsed laser methods for generation and detection of OH, we have reinvestigated the kinetics of the title reaction at pressures of 22–743 Torr (1 Torr = 1.333 hPa) and temperatures (273, 298, and 333 K) in pure N2 and in N2–H2O bath gases. In situ optical absorption measurements were used to rule out any bias due to NO2 or HONO impurities. Our rate coefficients (k1) in N2 bath gas are parametrized in terms of low-pressure (k0) and high-pressure (k∞) rate coefficients and a fall-off parameter (FC) with k1,0N2 = 7.24 × 10–31 (T/300 K)−2.17 cm6 molecule–2 s–1, k1,∞ = 3.3 × 10–12 (T/300 K)−0.3 cm3 molecule–1 s–1, and FC = 0.53. Used with the “Troe” expression for termolecular reactions, these parameters accurately reproduce the current data in the fall-off regime and also capture literature rate coefficients at extrapolated temperatures. The presence of water vapor was found to enhance the rate coefficients of the title reaction significantly. The low-pressure limiting rate coefficient in H2O bath gas is a factor 5–6 larger than in N2, at room temperature (k1,0H2O = 4.55 × 10–30 (T/300 K)−4.85 cm6 molecule–2 s–1) indicating that H2O is much more efficient in quenching the association complex HONO* through collisional energy transfer. Based on measurements in N2–H2O mixtures, a parametrization of k1 including both N2 and H2O as third-body quenchers was derived. Neglecting the effect of H2O results, e.g., in an underestimation of k1 by >10% in the tropical boundary layer.


INTRODUCTION
Nitrogen monoxide (NO) is a short-lived intermediate involved in a variety of chemical reactions throughout the Earth's atmosphere, 1,2 where it is quickly oxidized to NO 2 by reaction with O 3 , 3 peroxy radicals, 4 NO 3 , 5 and halogen oxides. 6 During the day, NO 2 is rapidly photolyzed back to NO so that a photostationary state between NO and NO 2 evolves. NO and NO 2 are together referred to as NO x , a critical component in the photochemical formation of ozone and smog in the lower atmosphere 1 and in the destruction of O 3 in the lower stratosphere. 7 Both NO and NO 2 can also be oxidized by reaction with OH in termolecular reactions forming nitrous (HONO) and nitric acid (HNO 3 ): During the daytime, HONO is photolyzed to OH + NO with a lifetime of ≥1 h 8 and may represent a significant source of OH in some environments, especially at sunrise. Apart from its formation in R1, additional sources of HONO include heterogeneous or photochemical reactions of NO x and other reactive nitrogen compounds on various surfaces, emission from soil, and the photolysis of particulate nitrate. 9−11 Termolecular reactions, which involve formation of an activated association complex whose relative rate of dissociation back to reactants and collisional quenching determine the effective rate coefficient, are pressure (and temperature) dependent. Such reactions often demonstrate "fall-off" behavior, and the Troe formalism 12 has been widely adopted to parametrize the rate coefficients in terms of high-and lowpressure limiting rate coefficients (k ∞ and k 0 , respectively) and a broadening factor (F C ) to characterize the transition regime in between. Recently, we presented measurements of rate coefficients for the termolecular reaction of OH with NO 2 and SO 2 under fall-off conditions at temperatures prevalent from the Earth's surface to the lower stratosphere. 13−15 For the title reaction, several experimental data sets 16−32 were obtained from the 1970s to 1990s, mainly at low pressures in He and Ar bath gases to aid detection of OH. Although highly desirable for the purpose of deriving atmospherically relevant rate coefficients, data sets in N 2 at conditions relevant for the lower atmosphere (pressures up to 1 bar air) are sparse. 22,27,32 Figure 1 presents a comparison between values of k 1 recommended by the IUPAC 33,34 and NASA 35 evaluation panels at different altitudes in the Earth's atmosphere (i.e., at different temperatures and pressures). The largest differences are seen for the lower atmosphere (especially in the planetary boundary layer), with better agreement in the stratosphere at low temperatures and pressures. IUPAC and NASA derived similar values of k ∞ (based on high-pressure measurements in He bath gas) and for k 0 based on different studies [19][20][21][22]24,26,27 in which N 2 was used as a third-body. To some extent, the different rate coefficients can thus be attributed to the broadening factors chosen: 0.6 by NASA and 0.81 by IUPAC.
Previous experimental work in different bath gases 18,19,21,22,25,36 elucidated the different collisional transfer efficiency of various third-body quenchers for the title reaction. In particular, H 2 O was found to be a more efficient third-body than larger molecules with more vibrational degrees of freedom such as SiF 6 and CF 4 . 22 The influence of H 2 O on k 1 was also highlighted in a recent study, 37 which explored the role of water clusters at very low temperatures (60−135 K) in a Laval nozzle expansion. Our recent studies on the reactions of OH with NO 2 and SO 2 14,15 revealed that HNO 3 /NO 2 and H 2 SO 4 / SO 2 ratios in some parts of the atmosphere could be significantly modified by the presence of H 2 O.
The goals of this experimental work are 1) to quantify the impact of H 2 O as a third-body quencher on the title reaction, 2) to derive accurate values of k 1 in the "fall-off" regime in N 2 bath gas, and 3) to provide a parametrization of k 1 suitable for modeling R1 throughout the atmosphere, thereby reducing uncertainty in this important rate coefficient.  13,38 and thus, only a brief description is provided here. The reactions took place in a jacketed, cylindrical quartz reactor with a volume of ∼500 cm 3 the temperature of which was controlled by circulating a 60:40 ethylene glycol−water mixture through an outer jacket. The temperature at the center of the reactor was measured by inserting a J-type thermocouple before and after each experiment. The pressure in the reactor and optical absorption cells (see below) was monitored by capacitance manometers (MKS) with ranges of 100 and 1000 Torr (1 Torr = 1.333 hPa). The experimental pressure was adjusted by varying the total flow rate and pumping speed. The total volume flow rate was varied to maintain an average linear velocity of ∼8−9 cm s −1 in the reactor at all experimental temperatures/pressures. The linear velocity at the center of the flow is likely to be larger (by up to a factor of 2 for laminar flow) than 8−9 cm s −1 , and as the 0.8 mm diameter laser beam propagates at right angles to the gas flow, we can be certain that photolysis occurs in a fresh gas mixture at each laser pulse (operated at 10 Hz).

EXPERIMENTAL SECTION
OH radicals were generated by photolyzing H 2 O 2 (R3) at a wavelength of 248 nm using a KrF excimer laser (COMPex 205F, Coherent).
OH radicals were excited at 282 nm (A 2 ∑ (ν = 1) ← X 2 Π (ν = 0)) by a YAG-pumped dye laser, and the subsequent OH fluorescence was detected by a photomultiplier screened by a 309 nm interference filter and a BG 26 glass cutoff filter. The delay between the triggers of the photolysis and probe lasers was scanned using a digital delay generator. Time-dependent OH profiles (one laser pulse per data point) were obtained by accumulating the fluorescence signals using a boxcar integrator; 20−50 successive profiles were averaged to improve the signal-to-noise ratio. The photolysis laser fluence was measured by a joule meter placed behind the exit window of the reactor, and the shot-to-shot variation in the intensity of the dye laser was monitored by a photodiode. Each OH decay profile was composed of 20 points before the excimer laser was triggered (to determine the background signal) and 100 points after the trigger of the excimer laser for use in deriving the decay kinetics. 2.2. Online Optical Absorption Measurements. In our previous studies of atmospherically important, termolecular reactions involving the OH radical, 13,15 the concentrations of the excess reactants (SO 2 and NO 2 ) were accurately measured through in situ optical absorption techniques. NO displays several resolved absorption features in the VUV 39 but the more accessible features at 205, 215, and 226 nm are weak and do not coincide with the wavelengths of the atomic line sources available (Hg lines at 185, 254, and 365 nm or Zn at 214 nm) Figure 1. Ratio between rate coefficients, k 1 , derived using the IUPAC and NASA parametrizations at different altitudes in the atmosphere. The pressures and temperatures at each altitude were calculated using parameters given in an Earth atmosphere model (https://www.grc. nasa.gov/www/BGH/atmosmet.html).
The Journal of Physical Chemistry A pubs.acs.org/JPCA Article or over the wavelength range (∼230−700 nm) covered by our long-path absorption cell equipped with halogen and deuterium lamps. Compared to NO 2 and SO 2 , which have affinity for surfaces, NO is easy to handle and has no losses in flow controllers, and diluted samples can be prepared with high accuracy. In this study, the concentration of NO was derived from its partial pressure in a supply canister, its partial flow rate into the reactor, and the total pressure and temperature. The mass flow controllers were freshly calibrated using a Gilibrator. The purity of the NO sample was checked using an optical absorption cell (l = 110 cm) located upstream of the reactor. Light from a deuterium lamp was passed through the cell 8 times (resulting in an optical length of 880 cm) and detected by a low resolution (Δλ = 2 nm) spectrograph (Ocean-Optics USB 2000). Absorption measurements between 250 and 600 nm were inspected for absorption features from NO 2 and HONO. The minimum absorbance that could be detected was 5 × 10 −4 at 420 nm, which, using a cross section of 6 × 10 −19 cm 2 molecule −140 for NO 2 implies a maximum concentration of 2 × 10 12 molecules cm −3 . This is a factor >100 less than the concentration of NO typically used in the experiments (3−20 × 10 14 molecules cm −3 ) and (as the rate coefficients for reaction with OH are similar) implies that NO 2 impurity does not significantly bias the loss of OH. Similarly, the characteristic absorption features of HONO at 354, 368, and 384 nm 41 were not observed, and an upper limit to its concentration could be established, once again excluding a significant bias to the data as a result of the reaction of OH with HONO.
A second (single-pass) optical absorption cell (l = 34.8 cm) equipped with a low-pressure 185 nm Hg lamp was located downstream of the reactor to measure water concentrations in the experiments using N 2 −H 2 O bath gases. An absorption cross section of σ H 2 O (185 nm) = 7.14 × 10 −20 cm 2 molecule −142 was used to retrieve water concentrations, with the pressure and temperature difference between the reactor and the 185 nm cell taken into consideration.
2.3. Chemicals. Nitrogen (N 2 , 99.999%) was supplied by Air Liquide and used without further purification. Hydrogen peroxide (H 2 O 2 , AppliChem, 35%) was vacuum distilled to >90 wt % purity. Distilled water (Merck, liquid chromatography grade) was degassed before use. Two different NO−N 2 mixtures were used for the experiments: one commercial mixture (nominal mixing ratio of 5%) was supplied by Air Liquide, and the other was self-made with 2.75 ± 0.05% NO. The self-made mixture was made using NO (99.9%, purchased from Air Liquide) following fractional distillation to remove impurities such as NO 2 and other nitrogen oxides. The uncertainty in the mixing ratio is based on a conservative estimate of the accuracy of pressure gauges used to make the mixture.

RESULTS AND DISCUSSION
3.1. Rate Coefficients (k 1 ) in N 2 . Rate coefficients for the title reaction in N 2 were measured at three different temperatures (273, 298, and 333 K) over the pressure range of 22−743 Torr. In all experiments, the OH concentrations were kept sufficiently low (at the level of 10 11 −10 12 molecules cm −3 ) in comparison to [NO] (3−20 × 10 14 molecules cm −3 ) to satisfy pseudo-first-order conditions so that the OH decay could be described by where [OH] 0 and [OH] t are the OH concentrations at time 0 and t, respectively, after the photolysis laser pulse. k′ (in s −1 ) is the pseudo-first-order rate coefficient defined as where k 1 is the bimolecular rate coefficient (in molecules cm −3 ), and k d (in s −1 ) accounts for OH removal through  ) as <5% as the NO−N 2 mixture was prepared as precisely as possible, and all the flow controllers were calibrated prior to the experiments. Overall, an uncertainty of 8% was estimated for k 1 .
As mentioned in the Experimental Section, two NO−N 2 mixtures were used for the measurements. The first set of experiments was carried out using the bottled, commercial mixture, and the second set was carried out using our self-made mixture. The commercial mixture was not a primary standard, and thus the mixing ratio of NO was not sufficiently wellknown to derive accurate rate coefficients. To obtain the exact NO concentration in the commercial (nominally 5%) mixture, measurements were performed under identical conditions using the two mixtures. Values of (k′-k d ), are plotted as a function of [NO] in Figure 4(a), in which the closed and open symbols represent measurements using the self-made and the commercial mixtures, respectively. The solid lines are the linear regressions for the (k′-k d ) measurements (in s −1 ) with the selfmade 2.75% NO mixture, which lie consistently above the data points obtained using the commercial mixture, indicating that the true NO concentration in the Air Liquide bottle should be lower than the nominal value. By systematically varying the mixing ratio of the commercial sample (using correction factors between 1 and 1.2) and refitting the data, we derived the best fit to the entire data set (i.e., the minimum standard deviation in the difference between the open symbols and solid lines in Figure 4). As shown in Figure 5, a correction factor of 1.086 (i.e., the true NO mixing ratio in the commercial sample is 4.60%) gives the best result. Figure 4(b) plots (k′-k d ) for all data obtained under identical conditions (both NO samples) when this correction is applied. Figure 6 displays values of k 1 measured in N 2 bath gas as a function of the N 2 concentration (N 2 pressure was 22−744 Torr) at three different temperatures (273, 298, and 333 K). The solid lines are global, least-squares fits according to the Troe formalism 12 for termolecular reactions where k 1,0 N 2 (in cm 6 molecule −2 s −1 ) and k 1,∞ (in cm 3 molecule −1 s −1 ) are the high-pressure and low-pressure limiting rate coefficients, respectively; T is the temperature in Kelvin; [M] is the molecular density in molecules cm −3 ; and n and m are dimensionless temperature exponents. The broadening factor F accounts for the lower rate coefficients in the fall-off regime compared to predictions by the Lindemann−Hinshelwood mechanism and is expressed as where N = 0.75−1.27 log F C , and F C is the broadening factor at the center of the fall-off curve.
To reduce the number of fit variables, and also because a relatively small temperature range is covered by the current measurements, we fix k 1,∞ and its temperature dependence to values obtained in experiments in He at pressures up to 150 bar 28 that indicated that k 1,∞ is ∼3 × 10 −11 cm 3 molecule −1 s −1 with the temperature dependence (m = 0.3) derived from measurements at 250, 298, and 400 K. 30 Hence, only the parameters k 1,0 N 2 , its temperature dependence (n), and F C are allowed to vary.
The results are summarized in Figure 6 (solid lines) and in Table 2 where we also list the values preferred by IUPAC and NASA. In the Supporting Information, we also list and discuss the results obtained when different (or no) constraints to the fits are used. In summary, the fits obtained when fixing k 1,∞ or when freely varying all parameters are of similar quality. However, the values of k 1,∞ , derived by freely varying all parameters are significantly lower than the results of high pressure experiments and have a strong negative temperature dependence, which reflects the fact that our data (in the fall-off region) do not define the high-pressure limiting rate coefficient well. The value of k 1,0 N 2 = 7.24 × 10 −31 (T/300 K) −2.17 cm 6 molecule −2 s −1 that we obtain is in good agreement with those preferred by IUPAC and NASA (see Table 2), although the value of F C = 0.53 is substantially lower than the calculated value of 0.81. We note that fixing F C to 0.81 and using the IUPAC parameters for k 1,∞ and m preclude a good fit to our data set (see discussion in the SI).
3.2. Comparison with Previous Measurements and Parametrizations for N 2 Bath Gas. Figure 7 presents a comparison of the present and previous measurements of k 1 in N 2 at around 298 K, our parametrization (Table 3) and the IUPAC and NASA evaluations at the same temperature.
Over the fall-off regime, most literature data sets obtained in N 2 were obtained at pressures well below 1 bar. 22,23,27,29 The current measurements and parametrization agree well with the data from Anastasi and Smith 23 and Donahue et al., 29 while the data sets reported in Overend et al. 22 and Sharkey et al. 27 lie slightly below and above our measurements, respectively, at   27 ), and both our new parametrization and the NASA evaluation reproduce the measurements of k 1 at 233 and 405 K, while the IUPAC parametrization results in higher values, especially at 233 K ( Figure S6). The rate coefficients reported by Sharkey et al. 27 at 216 K are larger than the parametrized rate coefficients, and their values at 298 K are also larger than reported in all other data sets (see Figure 7), which indicates a systematic bias related to their determination of the NO concentration. Figure 7 (and Figure S5) shows that the parametrization derived in this work converges with those of the evaluation panels, particularly NASA, at low pressures. 23,27,29 Values of k 1,0 derived at low pressures using the discharge flow technique 19−21,24 vary greatly (from 5.8 × 10 −31 to 15 × 10 −31 cm 6 molecule −2 s −1 ) which might be related to experimental difficulties including, e.g., correcting for OH wall losses and axial diffusion, and these data are not represented in Figure 7.    Figure 4) and the linear regressions (solid lines in Figure 4) through data points obtained with the self-made NO mixture as a function of the correction factor for the NO mixing ratio in the commercial sample.   The Journal of Physical Chemistry A pubs.acs.org/JPCA Article and 25% at 298 and 333 K to avoid condensation of water in any part of the reactor or optical cell. In all experiments, the fluctuation of the total pressure was <1% so that the resulting influence on the measured k 1 was less than 1%. Figure 8 plots values of k′ as a function of the NO concentration in four bath gases containing different amounts of water vapor at 298 K and documents an increase in the slope of the linear regression (i.e., in k 1 ), with the concentration of water. At the highest water vapor concentration used (2.9 × 10 17 molecules cm −3 ), k 1 increases by around 60% compared to the value obtained in pure N 2 at this pressure and temperature.
Values of k 1 obtained in N 2 −H 2 O bath gases at 50 Torr and at three different temperatures are plotted against x H 2 O in Figure 9. The increasing value of k 1 with x H 2 O indicates that H 2 O is a more efficient third-body quencher than N 2 for the title reaction and the effect of water on k 1 is also dependent on the temperature (largest slope at the lowest temperature). To evaluate the role of water in OH + NO kinetics and to derive a parametrization for k 1 , the following equations are used to analyze the data = + []    The solid lines are fits to eq 5 and eq 6 with k 1,0 N 2 , n = 2.17, k 1,∞ , m, and F C constrained using parameters obtained in Method 4 (Table S1). The resulting parameters in H 2 O bath gas are k 1,0 H 2 O = 4.55 × 10 −30 cm 6 molecule −2 s −1 and o = 4.85. The dashed lines are the corresponding fits when using k 1,0 N 2 , n = 2.17, k 1,∞ , m, and F C constrained using parameters obtained in Method 1 (Table S1) The Journal of Physical Chemistry A pubs.acs.org/JPCA Article Adopting the "dry" parameters obtained in pure N 2 (k 1,0 N 2 , n, k 1,∞ , m, and F C ) using Method 1 or Method 4 (listed in the first and fourth row of Table S1), a global, least-squares fit to the N 2 /H 2 O data set results in k 1,0 H 2 O = 3.81 × 10 −30 (T/300 K) −4.19 cm 6 molecule −2 s −1 (Method 1, dashed lines in Figure 9) or k 1,0 H 2 O = 4.55 × 10 −30 (T/300 K) −4.85 cm 6 molecule −2 s −1 (Method 4, solid lines in Figure 9). While the differences in the fits obtained using Method 1 and Method 4 are slight at 333 and 298 K, the use of Method 1 results in a poorer fit to the data at 273 K, which is (at least partially) due to the use of a larger value of k ∞ . For the purpose of constraining the fit to the data of the H 2 O−N 2 experiments, the accurate characterization of k 1 at low pressures is of primary importance, and the correct derivation of k 1,∞ is less essential. As the rate coefficients at 50 Torr are far from k 1,∞ and because the use of parameters obtained using Method 1 to constrain the fit gives the best fit, we prefer k 1,0 H 2 O = 3.81 × 10 −30 (T/300 K) −4.19 cm 6 molecule −2 s −1 .
In both cases, it is clear that k 1,0 H 2 O (300 K) is a factor 5−6 larger than k 1,0 N 2 (300 K), similar to the results obtained in our studies of OH + NO 2 (+M) and OH + SO 2 (+M). 14, 15 Overend et al. 22 performed measurements in He−H 2 O mixtures where the H 2 O partial pressure ranged from 3 to 16 Torr over a total pressure of 20−30 Torr at 295 K. The results are displayed in Figure 10 which also plots our parametrized fall-off curves for k 1 in pure H 2 O and pure N 2 for comparison. In both bath gases, the current data and parametrizations lie above the rate coefficients reported by Overend et al. 22 whose data are significantly more scattered than those of the present study, which appears to stem from scatter in the plots of k′ versus [NO]. Overend et al. 22 analyzed their data with a twostep Lindeman scheme and concluded that the collisional energy transfer efficiency of H 2 O was a factor 8.3 greater than that of N 2 , somewhat larger than the value of 5−6 derived in this work.
Liessmann et al. 37 addressed the role of H 2 O in their studies of the title reaction in a Laval-nozzle expansion (61−135 K) at pressures close to 1 Torr and documented a significant increase in the rate coefficient (factors of 1.06 to 1.44) in the presence of H 2 O (at 3% of the total pressure). Such a large enhancement in the rate coefficient in the presence of just 3% H 2 O (i.e., x H 2 O = 0.03) is much greater than observed at the higher temperatures of the present study or than of Overend et al. 22 As discussed by Liessmann et al., 37 the supersaturation of H 2 O in the expansion favors cluster formation and the formation of OH(H 2 O) n , NO(H 2 O) n prior to reaction, and also formation of the cluster HONO(H 2 O) n may play a role in their experiments and explain the much larger effects they observed. In contrast to the Laval-nozzle experiments, low temperatures in the Earth's atmosphere are accompanied by low water−vapor mixing ratios, and the results obtained in the present study (and in that of Overend et al. 22 ) are relevant for estimating the impact of considering (or, conversely, neglecting) the enhancement of k 1 in the presence of H 2 O.
3.4. Implications for the Atmosphere. The discussion above indicates that H 2 O is a much more efficient third-body quencher than N 2 for the NO + OH reaction, and a simple calculation serves to illustrate the impact of water vapor on the rate coefficient of the title reaction in the atmosphere. Consider the tropical boundary layer with a typical temperature of 30°C (303 K), a total pressure of 1 bar (750 Torr), and a humidity of 100%. The major components (bath gases) of the air are 567 Torr N 2 , 151 Torr O 2 , and 32 Torr H 2 O. We assume that O 2 has the same quenching efficiency as N 2 , which is generally a very good approximation. Despite its lower concentration, the higher quenching efficiency of H 2 O contributes more than O 2 to the collisional relaxation of HONO* (and thus the rate coefficient). The current parametrization yields values of k 1 (1 bar, 303 K) = 6.17 × 10 −12 cm 3 molecule −1 s −1 if the impact of H 2 O is ignored and a >10% larger value of 6.86 × 10 −12 cm 3 molecule −1 s −1 when H 2 O is considered (using k 1,0 H 2 O = 3.81 × 10 −30 (T/300 K) −4.19 cm 6 molecule −2 s −1 ). At the same temperature and pressure, the parametrizations of the IUPAC and NASA panels (neither of which takes H 2 O into account) result in values of 9.36 × 10 −12 and 7.09 × 10 −12 cm 3 molecule −1 s −1 , respectively. The present data set and parametrization should be used to reassess the kinetic data for the title reaction and guide the IUPAC and NASA panels toward reaching consensus on their preferred values, especially at lower altitudes.

CONCLUSIONS
Rate coefficients of the title reaction NO + OH were measured at various temperatures and pressures (N 2 ) in the fall-off regime and used to develop a parametrization that accurately describes the present data and literature data sets even at temperatures outside the range of our measurements. Experiments in N 2 −H 2 O bath gases showed that water is a more efficient third-body quencher than N 2 by a factor of 5−6. The water effect was parametrized using a Troe type expression considering multiple bath gas components, which provides a comprehensive and reliable basis for atmospheric modeling. The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpca.2c02369.
Description of fitting constraints (Methods 1−5) and associated figures (Figures S1−S4), comparison of k 1 with evaluations ( Figures S5 and S6), NASA parametrization method for termolecular reactions, and parametrization of k 1 in N 2 −H 2 O bath gases using different values of F C for N 2 and H 2 O (PDF) Figure 10. Fall-off curves for k 1 in H 2 O and N 2 bath gases at 295 K. Solid lines are the current parametrizations based on Method 4 (see Table S1). Symbols are measurements reported by Overend et al. 22 The Journal of Physical Chemistry A pubs.acs.org/JPCA Article